발간논문

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The aging characteristics of Cu-1.8%Be-0.2%Co alloy have been studied by hardness measurement and transmission electron microscopy. The zone formed in the early stage of aging was considered to be beryllium rich GP I zone which is characterized by the continuous long streaks in the <100>, <110> directions in the selected area diffraction pattern. Further aging produced discontinuous streaks with the several intensity maxima. This stage may correspond to GP II zone. To explain the maximum density points, a model is proposed that a Be rich atomic layer lies in about every third plane of the matrix lattice planes. Aging at higher temperatures caused superlattice reflections of CuAu (I) type γ` to appear The final equilibrium precipitate was identified to be body centered cubic γ. GP I , GP II and γ` are fully coherent with the matrix. Furthermore the alloy shows the maximun hardness when GP II coexists with γ`.

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The microstructure and mechanical properties of 55 Nitinol and those with 1.0, 2.5, and 5.5 at. % cobalt substituted for nickel respectively have been investigated. First, alloys are melted by non-consumable electrode arc melting furnace, under helium. After several remelting, these alloys are cast in a high frequency induction vacuum furnace. Any other secondary phases by substituting Co for Ni are not observed. The values of hardness, yield strength, tensile strength, and compressive strength of alloys are increased by Co addition the maximum values occurring at 2.5% Co, and more Co substitution shows rapid decrease. The martensitic transformation temperatures are reported to vary within wide limits by changing the composition of alloy. Cobalt is an effective substitution element for lowering the transformation temperature. The relations between the transformation temperature and Co substitution were obtained as follows, M_S
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The aim of this experimental work is to find out the possibility of boronizing of spheroidal graphite cast iron (SGCI) in order to improve surface hardness and wear-resistivity. Samples with various silicon content were prepared in a Kryptol furnace and heat-treated to obtain the pearlitic or ferritic matrix. Liquid boronization in a molten bath containing borax, boron carbide and ammonium chloride was carried out with these samples having different silicon content and matrix. The boronized layer consisted of iron boride layer and boron-diffused layer, the former was mainly Fe₂B and the latter contained the precipitated phase of boro-carbides. The depth of the iron boride in the annealed matrix was increased to a certain extent rather than in the as-cast matrix, but its change with silicon content was not remarkable, Hardness of the boron-diffused layer was increased as the silicon content increases. In the case of high silicon content, graphite accumulated zone was formed beneath the iron boride layer, resulting in tearing off the iron boride layer. When as-cast structure containing cementite was boronized, elongated graphites appeared sporadically in the iron boride, causing the decrease of hardness. Both surface hardness and wear-resistivity of the boronized SGCI, except for high silicon contained, were improved remarkably.

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Two different types of martensite structure of Fe-Ni alloy (20%Ni and 32%Ni), one having retained austenite and the other no retained austenite, were prepared and then the effect of prior deformation applied below Ad point on the austenite start temperature (As) of these martensites was studied. The As of Fe-32%Ni alloy increased appreciably up to about 40% deformation and then showed a nearly constant value above 40% deformation, while As of Fe-20%Ni alloy showed a constant value up to about 40% deformation and decreased gradually with further deformation. The different feature in the effect of plastic deformation on As in two alloys is attributed to whether retained austenite is present or not: in the absence of retained austenite, the reversal reaction is controlled by nucleation of new austenite, and in the presence of retained austenite, it is controlled by its growth.

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The spheroidization of cementite in ternary Fe-C-Si alloy was investigated at temperatures of 600, 640 and 680℃ for times up to about 10^6 sec. The mean size and the number of particles per unit volume were obtained by quantitative metallography techniques, and analyzed by means of Lifshitz-Wagner model assuming diffusion-controlled mechanism. The effective diffusion coefficient was between the values for the diffusion of carbon and ron in ferrite and way clone to the coupled diffusion coefficients developed by Oriani and Li, Blakely and Feingold. The effective diffusivity is 5.6×10^(-11)㎠/sec at 680℃.

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The effects of cooling rate on the machinability and other mechanical properties of free cutting brass were examined through sand mold, metal mold and water-cooled metal mold casting respectively. When the cooling rate was rapid, the lead particles were finely dispersed throughout the matrix and the β-phases appeared to be needlelike, when the cooling rate was slow, the lead particles formed themselves into large spheroids mainly along the grain boundaries while the β-phases shaped into plates. The machinability and mechanical properties turned out satisfactory in the case of rapid cooling rate.

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This work was undertaken to study the effect of degree of working applied during the drawing on spheroidization of cementite in 0.74%C steel wire. The effect of drawing was evaluated by the particle size of spheroidized cementite. Particle size of spheroidized cementite was calculated by Fullman method, replacing the number of particles per unit area counted in photomicrograph. Specimens of steel wire were prepared by cold drawing from 4.0㎜ to 2.8, 2.0, 1.4 and 1.0㎜ in diameter, and annealed at temperatures between 640° and 730℃ for various times up to 45 hours, From this experimental results, it was found that the spheroidization of cementite was controlled by diffusion, and also the effective diffusion coefficients were increased with increasing amount of cold work. The activation energy for the spheroidizing was found to be 40 ㎉/mole. The results may be rationalized by the theory proposed by Liishitz-Wagner.

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The effect of cooling rate on transformation temperature, microstructures and microhardness of pure uranium and U-(650, 1500 ppm) Al alloy s was investigated. It was found that the transformation temperatures of both gamma-to-beta and beta-to-alpha decreased with the increase of the aluminum contents. The degree of the lowering of the transformation temperature was increased as the aluminum contents and cooling rate were increased. Grain size was decreased with increasing the cooling rate. An feathery or accicular structures were obtained when aluminum added alloys were severely cooled from gamma range. The precipitates of UAI₂ appeared with in grains or on the grain boundaries of alloys. The size of the precipitates decreased with the increase of the cooling rate. Microhardness measurement showed that hardness increases as the alumiunm contents and coding rate were increased. The increase of the hardness is supposed to be resulted from the size of precipitation arid lattice hardening due to the martensite transformation.

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